The technique for simultaneously removing flue gas pollutants by the non-thermal equilibrium plasma is one of the most promising comprehensive flue gas treatment techniques currently, which uses particles with high reactivity produced from gas discharge to react with various organic and inorganic pollutant molecules which are thereby decomposed and oxidized into harmless substances or easily treatable compounds. This technique has the characteristic of effectively, conveniently and non-selectively decomposing various pollutants by using simple equipment and small space, therefore it is adaptive to various working environments, and as a result, the application of non-thermal equilibrium plasma technique for simultaneously controlling pollutants in flue gas has drawn the universal concern at home and abroad, and the related techniques have been at the stage of commercial application based on plenty of basic principle researches and technical popularization. At present, the most used plasma flue gas treatments are electron beam process, pulse corona process and direct current corona process.
During the removal of pollutants by using the electron beam process, N2, O2, CO2, vapor and the like absorb most electron beam energy after the irradiation of the electron beam to produce various free radicals including OH, O, HO2 and other particles with high reactivity. The free radicals are capable of oxidizing SO2 and NOx in the flue gas to produce sulfuric acid and nitric acid which would then be neutralized by the pre-introduced ammonia to produce ammonium sulfate and ammonium nitrate. The Ebara Corporation in Japan started the research on the technique of irradiating flue gas by electron beam for desulfuration and denitration in 1970, cooperated with the Japan Atomic Energy Research Institute in 1972, treated heavy oil combustion waste gas and performed experiments on the scale of 1000 Nm3/h with the radiation effect of the addition of ammonia detected, the conditions for desulfuration and denitration stabilized and the byproducts, ammonium sulfate and ammonium nitrate, entrapped successfully in 1974, and cooperated with the Research Institute for Controlling NOx of Steel Industry, and performed experiments on the scale of 10000 Nm3/h of sintering waste gas furnaces in 1977, so that the dry technique was affirmed. However, the main gas molecules in the high-energy electron pair flue gas produced from the electron beam process can break down the chemical bonds thereof, and decompose and ionize gas molecules with high content, such as N2 and CO2, in the flue gas, causing energy waste and large energy consumption in the process. Furthermore, the problems of expensive electron gun, shot longevities of electron gun and target window, complex equipment structure, large space, ammonia escape, shielding and protection of X-ray and the like are existing.
The pulse corona process may fast accelerate electrons within nanosecond-scale time, so that the electrons obtain larger energy, while less energy can accelerate the more massive ions, so as to greatly reduce the energy waste caused by the ion acceleration; meanwhile, the pulse corona process may reach a higher voltage peak to form a higher discharge intensity, making the high-energy electron energy reach 5-20 eV. The pulse corona process has been being widely researched since 1980s. In 1986, Masuda proposed a pulse corona discharge process of replacing electron accelerator with pulse high-voltage power supply above tens of thousands of volts to produce plasma, and performed plenty of fundamental researches which showed that NO and SO2 were capable of effectively being oxidized into NO2 and SO3 by the pulse corona plasma chemical process. The Research Institute of Dalian University of Technology for Electrostatic and Special Power Supplies created a desulfuration device of 3000 Nm3/h, in 1996; Lee et al. created a large reaction device for simultaneous desulfuration and denitration by using pulse plasma technique, with the capacity of 42000 Nm3/h, and the desulfuration and denitration rates respectively of 99% and 70% by using ammonia free radical and propylene free radical under the energy consumption condition of 1.4 Wh/m3, in 2003. The energy consumption of the pulse corona process was only 50% of that of the electron beam process, but the pulse corona process still took effect and apply to ionize gas molecules, such as N2 and CO2, in the flue gas, resulting in energy waste. In addition, during the actual application of the electron beam process or pulse corona process, ammonia gas is usually added as absorbent and incapable of complete reaction, resulting in leakage and pollution to the surroundings after being discharged into the air.
The direct current corona discharge is a discharge form produced by the non-uniform electric field distribution between electrodes under the action of direct current and high voltage. The direct current corona discharge shares the similar working principle as the pulse corona discharge's, and has the advantages of simple discharge power supply, wide application of high-power direct current power supply in the electrostatic precipitation field, and mature and reliable technique. Chang et al. constructed a flue gas purification demonstration project of 1000-1500 Nm3/h for desulfuration and denitration of flue gas with direct current corona ammonia injection free radical shower with the removal rates of NO and SO2 of 75% and 99% respectively, and the energy consumptions of 125 g NO/kWh and 9 g SO2/kWh respectively.
In the pulse and direct current corona discharge pollutants removal systems, if the conventional discharge electrode structures are used, such as wire-cylinder structure and wire-plate structure, the corona zone of flue gas direct current corona discharge is smaller and only limited to the proximity of wire electrodes, and the discharge current is weaker, so that the pollutants removal effect is poor. If multi-pin and one-plate discharge structure is used, although the corona discharge zone and discharge current are increased, the energy consumption is larger.
For the deficiencies above, someone proposed to use the nozzle-type discharge electrode. China patent, Public Number CN1600408A filed on Mar. 30, 2005, discloses a discharge nozzle electrode for the direct current flue gas treatment with corona discharge, including a main pipe open at one end and sealed at the other end, and multiple nozzles symmetrically arranged on the main pipe. The additive gases introduced in the discharge nozzle electrode are ionized in the corona zone to produce plenty of active substances capable of reacting with the pollutants, so as to remove the contaminations. In addition, the different additive gases are capable of being decomposed in the corona zone to produce different free radicals so as to be selected correspondingly according to the objects to be treated in the flue gas. With the electrode design, the air velocity in the nozzles is faster than the flue gas velocity in a reactor, so that the free radicals produced can be better transferred and contact with and oxidize the contaminations. The electrode is advantaged in that the additive gases are ionized to a great extent in the corona zone nearby the nozzles, and the flow of the gases at the nozzle mouths can make the corona discharge more stable, so as to produce stable streamer corona across the whole discharge air gap. However, the additive gases are sprayed out from the ends of the nozzles, and the experimental result of the flow ionization zones is shown in FIG. 7, from which it can be seen that the flow ionization zones 13 are so concentrated, the free radicals 14 produced are mainly concentrated between the nozzles of electrodes 2 and upper and lower electrode plates 11, and the flue gas through its channel 12 is easy to pass through the gaps between the electrodes, without reacting with the free radicals produced by the decomposition in the corona zone, thereby influencing the treatment effect. If the electrodes are arranged more densely, the energy consumption of the system will be considerably increased. In addition, such hollow electrode is difficult to process and has high cost, upon which the investment on the equipment for engineering application will be obviously increased.